cftw: Allow directories to call the user callback

This provides a few benefits:

- This will allow searching for directories by name.
- Future commits will allow to remove files and directories, so this
  change was also required for cftw.

cftw.c

@@ -57,7 +57,12 @@ int cftw(const char *const dirpath, int (*const fn)(const char *,
             fprintf(stderr, "%s: stat(2) %s: %s\n",
                 __func__, path, strerror(errno));
         else if (S_ISDIR(sb.st_mode))
-            ret = cftw(d.str, fn, user);
+        {
+            if ((ret = cftw(d.str, fn, user)))
+                ;
+            else if ((ret = fn(d.str, &sb, user)))
+                ;
+        }
         else if (S_ISREG(sb.st_mode))
             ret = fn(d.str, &sb, user);
         else

main.c

@@ -768,6 +768,9 @@ end:
 static int add_length(const char *const fpath, const struct stat *const sb,
     void *const user)
 {
+    if (!S_ISREG(sb->st_mode))
+        return 0;
+
     unsigned long long *const l = user;
 
     *l += sb->st_size;